Characteristics of the dual-polarization structure and raindrop size distribution of a squall line in Shandong

Based on the Jinan dual-polarization radar data combined with raindrop size distribution (DSD) data measured by the Precipitation Phenomenon Instrument (PPI) and conventional observation data, the characteristics of the dual-polarization structure and DSD of a squall line occurred on 1 August 2021 in Shandong province are investigated. Results are as follows: The squall line is characterized by a broad canted updraft area at its leading edge and a strong downdraft area at low levels. Newborn convective cells were continuously triggered and rapidly grew in the updraft area, and achieved intense development by merging with the main echo, whereas the old convective cells in the rear part faded out. These processes continuously cycled within a short time period, which supported the squall line to develop, maintain and move forward rapidly. At the leading edge of the squall line, newborn convective cells were dominated by liquid particles with a few larger particles; in the merging convective cells, the concentration of liquid particles dramatically increased, which induced higher Z_DR and K_DP columns; at the rear part of the squall line, ice-phase particles melted distinctly as falling down below the freezing level, leading to the increases in Z_DR and K_DP. In particular, the increase of K_DP near the surface was more pronounced, which indicted high concentration of liquid particles that resulted in heavy rain. The most obvious DSD feature for the heavy rain is the high concentration of drops especially the drops smaller than 2 mm. The rain primarily depended on the particles sized 1—3 mm and interfered by a few extra-large particles in the early strengthening stage of rainfall. Nevertheless, particles with 2—4 mm diameter contributed most to the heavy rain in the most vigorous stage of rainfall. The dual polarization parameter with decreasing Z_DR and increasing K_DP at low levels of the squall line, as rain intensified, corresponded well to the variation of DSDs with significantly increasing particle concentration and distinctly decreasing extra-large particles (>6 mm).